Method of treating low-carbon steel and the like



1 Jan. 29, 1929.

v A. FRY

METHOD OF TREATING LOW CARBON STEEL AND THE LIKE Filed Sept. a, 1925 Patented Jan. 29, 1929.

UNITED STATES PATENT OFFICE.

ADOLF FRY, OF ESSEN, GERMANY, ASSIGI VOR '10 FRIED. KRUPP A KT IENGESELI L- SCHAIT, F ESSEN-ON-THE-RUHR, GERMANY.

'mETnon or TREATING Low-cannon STEEL ANDTHE LIKE.

Application filed September 8,1926, Serial No. 134,325, and in Germany July 7, 1926.

A method of treating low carbon-steel and the like.

It is known that low carbon-st eel as employed for instance in manufacturing steam boilers, profile iron, tanks for the chemical industry etc. is generally very sensitive to the aged state and to the, chemical influences of liquids, for instance soda solution, or of gases and vapours. This sensibility appears for instance in that the original good tenacity of the notched bar or toughness decreases to Very low values, when the steel 1s subjected to a treatment which produces the said aged state particularly in connection with the mentioned chemical influences. The

said treatment is to be understood to consist for instance in deforming the material at about room-temperature and subsequently leaving it during -a greater length .of time to itself at about room-temperature or subsequently shortly heating the material up to a temperature of about 200 C. during about half an hour. A production of the aged state is also obtained when low carbon-steel is deformed at temperatures .between 150- and 500 0., and in this case also without a subsequent state of rest or reheating to about 200 C.

A steel which is sensitive to the aged state is spoiled in a particularly high degree when it is subjected to the influence of several liquids, as for instance soda solution, or of gases and vapours, as for instance ammonia gas.

' In this case not only the tenacity of the notched bar decreases to very small values, but at the same time serious disturbances in the texture of the steel, especially in the grain boundaries will arise. I

My patent application Ser. No. 134,324, 40 filed Sept. 8th, 1926 already has for its object a steel which is not sensitive to the aged state merely owing to the method of smelting it. But no method was hitherto known to free a low carbon-steel which is sensitive to aging, from this sensibility. By low carbon steel may be understood an alloyed or not alloyed soft ingot iron containing up to 0.4%C, as well as wrought iron.

Now the object of my invention is to provide a method according to which it is made possible to convert a steel which is sensitive to the aged state, by means of a heat treatment in the solid state, into a condition in which it is not longer sensitive to aged state.

This object is obtained by quenching the low carbon-steel at a temperature higher than about 650 C., which is the lowermost temperature at which the quenching can take place to produce the desired result, the temperature being lower or higher, according to whether the steel is alloyed or not. By a subsequent reheating of the quenched steel at temperatures which are lower than the quenching temperatures and, according to the composition of the steel, may amount to 750 (1. if the quenching temperature is above 7 50, textures are produced which are distinguished by a fine repartition of the particles of the texture and therefore offer a particularly good resistance to chemical influences of solutions, gases and vapours. For asoft, not alloyed ingot iron containing 0.1% of carbon, both the admissible lowermost temperature at which the quenching and the highest ten'iperature at which the reheating have to-be executed, is about 730 0., but both a small hysteresis of the phenomenon, especially with a simultaneous presence of alloy constituents, and a displacement of the limit temperatures dependent on the composition must be presumed, according to the composition of the material to be treated. The working treatment can take place before or after-the said heat treatment.

The quenching and reheating has been often employed withsteels having a higher percentage of carbon, and this treatment is known in the art. The object of this method is always to impart to the material a high yield point as well as high strength and duetility. The said methods are however only used in treating carbon-steels having a content of carbon of about 0.5% or more. With soft, not alloyed steel they are not employed in the industry, as it had been hitherto pre- 95 sumed that no important technical improvementis imparted to such a steel by quenching and drawing it. In particular, it was not known that the sensibility to the aged state of low carbon-steel is obviated by treating it in the described manner.

The high efficiency of my present method is to be seen from the following table and the accompanying curves. It will be seen from these numeral values and curves thatthe low carbon-steel, which in itself is sensible to the aged state, is highly improved by means of the present method, so that it becomes free from any sensibility to the aged state: The tenacity of the notched bar of a test piece I which has been heated to 900 0., stretched by Ill 5% of its length and brought to an aged state, is 2.7 mkg/c m whilst a test piece which hadprimarily been quenched at 930 C., then reheated to 630 0., and subjected to the same treatment producing the aged state, reaches a tenacity which is more than 31.6 mkg/cm For testing the tenacity standard Charpynotched bars of 30 mm". 160 mm having an annular notch, were used. The test took place at a temperature of +20 C. The numeral values present average values, each resulting from three shock tests of the notched bar. The composition of the material was the following: C=0.08%, Si=a trace, Mn=-.43%,

P=0.014%, 00am, N =0.14%, or 0.02%, N =0.004%, O 0.041"0.

Tenacity of the notched bar in mkglem Stretched Heat treatment Stretched stretched by about Not by 5% of by of of stretched lengt-h lengt-h length and in and in and in aged state aged state aged state Heated$ h. u to 900 C. and 00010 in air.-.. 18. 6 2. 7 1. 7 2. 2 Heated 34 11. up to 830 C. and quenched in water 28. 0 12. 2 Heated $4 11. up to 930 C. and quenched in water 25.2 9 8 Heated $4 h. up to 930 0., quenched in water and reheated up to 430 C. during 3 h 27. 3- 11.9 8. 7 13. O Heated% h. up to 930 0., quenched in water and reheated up to 630 C. during 3 h 33 31. 6 32.2 17.8

bar which may be attained by means of the heat treatment according to my invention, and this with a stretching of 5% of length and with the steel in the aged state. The curve denoted by A illustrates the tenacities' of the notched bar of annealed ingot iron (that is: testpieces of iron annealed for half an hour at 930 (1., and cooled in air), which tenacities are obtained, if the test pieces are quenched at t C. The cur-ve'shows that in this case the most favourable values are obtained at a quenching temperature of more than 730 C. The curve denoted by B shows the values of the said tenacity with test pieces .to ageing and less liable to destruction of texture by the influence of liquids, gases, and vapors, which consists in subjecting the low carbon steel to a quenching action at a temperature of more than 650 C.

2. A method in accordance with claim 1 in which subsequent to said quenching action the steel is subjected to a drawing operation at a temperature below the temperature of the quenching operation,butnot higher than A The foregoing specification signed at C0- logne, Germany, this 23rd day of August, 1926. I

ADOLF FRY. 

